Why Polarized Light Is AffectedFigure 100 A polaroid filter
allows light through only if the
light is polarized at the same
angle as the filter.
33.3 Why Polarized Light Is Affected
So why do chiral molecules affect only polarized light, and not unpolarized? Well, theydo
affect unpolarized light, but since the rays have no particular orientation to one another, the
effect can not be observed or measured. We observe the polarized light rays being rotated
because we knew their orientation before passing through the chiral substance, and so we
can measure the degree of change afterwards.
What happens is this; when light passes through matter, e.g. a solution containing either
chiral or achiral molecules, the light is actually interacting with each molecule’s electron
cloud^1 , and these very interactions can result in the rotation of the plane of oscillation for a
ray of light. The direction and magnitude of rotation depends on the nature of the electron
cloud, so it stands to reason that two identical molecules possessing identical electron clouds
will rotate light in the exact same manner. This is why achiral molecules do not exhibit
optical activity.
In a chiral solution that is not a racemic mixture, however, the chiral molecules present in
greater numbers are configurationally equivalent to each other, and therefore each possesses
identical electron clouds to its molecular twins. As such, each interaction between light
and one of these ’majority’ molecule’s electron clouds will result in rotations of identical
magnitudeanddirection. Whenthesebillionsofbillionsofinteractionsaresummedtogether
into one cohesive number, they donotcancel one another as racemic and achiral solutions
tend to do - rather, the chiral solution as a whole is observed to rotate polarized light in
one particular direction due to its molecular properties.